In Search of the Perfect (Sous Vide) Egg

People have been cooking chicken eggs since time immemorial, and an often-used insult for someone who isn’t a very good cook is to say, “She doesn’t even know how to boil an egg!”[1]

But in fact, boiling an egg involves several different competing phenomena, and it really isn’t all that easy. If you simply boil it until the yolk reaches the desired consistency, then almost invariably the white of the egg will be overdone and rubbery.

The reason, as Douglas Baldwin explained in a post to eGullet regarding sous vide eggs[2], is that there are four important proteins in an egg, and they denature or thicken at different temperatures:

“The important temperatures and proteins when cooking an egg in its shell are:

143°F (61.5°C): the protein conalbumin denatures and causes the egg white to form a loose gel

148°F (64.5°C): the protein livetin denatures and causes the egg yolk to form a tender gel

158°F (70°C): the protein ovomucoid denatures and causes the egg white to form a firm gel (the egg yolk also coagulates around this temperature)

184°F (84.5°C): the protein ovalbumin denatures and causes the egg white to become rubbery.”

The solution, according to Baldwin, is to cook eggs at a temperature between 70°C/158°F and 84.5°C/184°F, so that the conalbumin and ovomucoids (but not the ovalbumin) will be denatured, for a time to heat the yolk to a temperature around 64.5°C/148°F, yielding a soft-boiled egg with a firm by not rubbery egg white and a soft to creamy yolk. Because the time required for the center of the egg to come up to temperature is a function of the size of the egg, Baldwin has computed a table for eggs of various sizes, “In-Shell Egg Heating Times in a 75°C Water Bath using Circumference.”[3]

And that method, without the mathematical sophistication but simply based on experience, has been used and taught by almost every successful cook book author from Julia Child to Robuchon, including the noted cooking-science writer Harold McGee. Even the estimable multi-volume Modernist Cuisine cookbook set by Nathan Myhrvold, et al, repeats the conventional wisdom that it is temperature, and not time, that determines the consistency of an egg. They all recommend boiling eggs for 3-6 minutes for a soft yolk, 6-8 minutes for a medium soft yolk, and 8-10 minutes for a hard yolk.

And yet as recounted by Martin Lersch[4], even Jules Vern had observed that on a fictional comet of suitable mass, water will boil at 66°C, and that an egg cooked at that temperature for a quarter of an hour will only be sufficiently cooked. The famed Japanese onsen eggs were originally cooked in a hot spring for about an hour.

But due to recent research by César Vega and Ruben Mercadé-Prieto entitled “Culinary Biophysics: on the Nature of the 6X°C Egg”[5] and available as a free PDF file, we now know that almost everything we had assumed about cooking eggs was wrong, and in fact thetexture of the egg yolk is the result of a combination of both the time and the temperature used — its thermal history, if you will.

Basically, an egg is made up of complex proteins, and proteins do not denature instantly, but instead the reaction takes place as a slow but steady change over many minutes or even hours, depending upon the temperature. The problem is that many of us, including myself, conflated the behavior of a cut of meat cooked sous vide with that of an egg. A rib-eye steak will hardly change at all in taste or consistency, whether you cook it for one hour or six hours, and we (or at least I) simply assumed that an egg would behave similarly. Wrong!

In a brilliant piece of culinary biophysics, Vega and Mercadé-Prieto carefully separated the egg yolk from the whites, and then measured the viscosity (consistency) of the cooked result through small deformation rheology as they changed the time and temperature.

But in an even more useful result for chefs throughout the world, in addition to listing the viscosity of the resulting egg yolks, they also compared them to the viscosity of a variety of thick edible fluids at 25°C, ranging from whipping cream to pancake syrup, to molasses, to sweetened condensed milk, and on through mayonnaise, chocolate pudding, honey, cookie icing, toothpaste, and Marmite®.

Finally, they plotted the total cooking time vs. holding temperature, from 59°C to 68°C, with lines that represent a desired texture in simple, easy to understand terms:

Now, at last, we have a means of controlling the consistency of the egg yolk through a very simple means. Simply cook the eggs in a temperature-controlled water bath for the desired time and temperature, using something like the Sous Vide Magic controlling a rice cooker filled with water, or a high-end immersion circulator, or even a Styrofoam cooler and tea pot of hot water, and then either serve the egg immediately or chill it for future use. For example, picking 63°C for a reasonable trade-off between the precision of graph and the cooking time, we can see that 50 minutes would yield a consistency of sweetened condensed milk, 80 minutes that of honey, 125 minutes that of cookie icing, and approximately 160 minutes that of Marmite. If those times are too long for your taste, pick a slightly higher temperature and adjust accordingly.

Ok, fine! But now, what about the whites?

Well, here we can go back to a much more basic cooking technique, simply boiling the eggs for a short period of time in order to set the white to the desired consistency, but not so long as to heat up the yolk; then chilling them back to room temperature before cooking the egg yolks to the desired consistency. The outer egg white (the thin albumin) that is in contact with the shell will set, while the inner white (the thicker albumin) that surrounds the yolk will not, and neither will the yolk itself. But when using this technique, it is important to cool the egg quickly after boiling it, so that the hot temperature does not diffuse to the center of the egg.

According to several reports, the boiling step to set the egg white can be done either before or after cooking the egg yolk with equivalent results, so long as the egg is chilled back down to room temperature before cooking the yolk. But it should be noted that it is the interior of the yolk that needs to be chilled, and not just the whites, so the water should be cold, and the egg left in it for perhaps 30 minutes before cooking the yolk.

In my experiments, pre-boiling the egg for two minutes left the white a little runnier than I would like for most applications, but three minutes was about right. (I should note that I live in Taos, NM at an altitude of 7000 ft or 2133 meters, where the boiling point of water is 99.9°C /199.3°F, neglecting barometric pressure fluctuations. If you are at sea level, a little less than three minutes might be required for the same results.)

The eggs were pre-boiled for two and three minutes, respectively, directly from the fridge, then allowed to cool down to room temperature in a water bath. Then they were cooked sous vide at 63°C/145°F for 60 minutes, 75 minutes, and 125 minutes, using two sous vide apparatus. Of the three different times, 63C for 125 minutes following a 3-minute pre-boil was clearly the best, at least from the standpoint of standing alone and looking “sexy.” A thin lining (1 or 2 mm) of egg white was left inside the shell, but the yolk with some white set around it separated cleanly. When the yolk was broken with a fork, the contents were somewhere between the consistency of honey and cookie icing — just like I wanted for my purposes, e.g., for serving the yolk on grilled asparagus, but maybe too thick for a poached egg on toast.

I probably could have used a paper towel to remove the little bit of white around the yolk, if desired, but in that case I would cook several eggs, in case I inadvertently broke the yolk, as I did when I tried it.

I haven’t run any experiments with different size eggs, medium through jumbo, but I doubt that it would make much of a difference. On the other hand, the difference in the proteins might make a considerable difference when switching from chicken eggs to quail, duck, or ostrich eggs. More experimentation is in order.

Now of course, this isn’t the only way to cook an egg. If you want a sunny-side-up fried egg with a crisp edge around it, or if you want a scrambled egg, or an egg custard, that is certainly your choice. Some people simply don’t like sous vide eggs, although perhaps they should try the above technique before ruling them out. Chacun à son gout!

Finally, a word of caution if you are using an immersion circulator, or a Fresh Meals Magic with a bubbler, or any other sous vide appliance with a built-in circulator. There is a non-trivial risk that an egg could crack during the cooking process, and especially so if they are bouncing around in the tank. If that happens, you are going to have a messy cleanup to deal with — particularly if any egg white coagulates inside the circulator. For that reason, I would recommend cooking the eggs in a Ziploc bag that is filled with hot water from the thermal bath to surround the eggs. You may want to suspend the bag from a skewer to facilitate filling it, as well as providing a means of fast and easy retrieval. The water in the bag will quickly equilibrate with the water in the sous vide container, but if an egg cracks or breaks it will be confined to the bag.

Enjoy!

[1] My apologies for being non-PC. Feel free to substitute “He or she ” as appropriate.

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